My invention relates to selective heating apparatus and more particularly to heating apparatus for soldering a plurality of spaced electrical elements.
Soldering has long been used for joining electrical elements in electronic apparatus. More recent developments in electronic circuit techniques have required arrangements for simultaneously soldering a large number of spaced electrical terminations, such as connector elements, flat, flexible cable and flexible circuits. One method for such batch soldering involves placing a soldering tool in contact with the surface of previously solder-coated spaced terminal members. Radiant energy is focused on the tool, which is heated thereby and the tool heat is transferred to the solder-coated members to cause solder flow. The solder operation is accomplished after cooling of the solder. The conversion of energy to radiant form and then to heat, however, is relatively inefficient and the apparatus for the conversion is generally large and bulky, whereby the radiant energy technique is not readily adaptable for use in high density electronic equipment.
Another arrangement for batch soldering uses a relatively long slender bar or wire which is placed in contact with the line of spaced terminal members to be soldered after the bar is heated by means of an electrical current directed longitudinally along the length of the slender bar. Although the heat is generated in close proximity with the surfaces to be soldered, the variations in resistivity along the slender bar produce unwanted variations in temperature. Additionally, high resistivity in a portion of the slender bar produces temperature instability problems which limit the length of the bar that may be used. Generally, a bar length of one to two inches is suitable. Longer slender bars result in tool failure or large gradients in temperature. Although the cross section of the slender bar may be varied to reduce large variations in temperature, the instability problems are inherent in the slender bar geometry. Consequently, only a limited number of spaced electrical terminations can be soldered in each operation. Presently, batch soldering of spaced electrical terminations may require a uniform temperature over a length of six or more inches, in which event the slender bar soldering tool is inadequate. It is therefore an object of the invention to provide economic apparatus for simultaneously and uniformly heating a large number of spaced electrical elements to provide soldered connections.